Aerogels describe a class of material based upon their structure, namely low density, open cell structures, large surface areas (often 900 m2/g or higher) and sub-nanometer scale pore sizes. Supercritical and subcritical fluid extraction technologies are commonly used to extract the fluid from the fragile cells of the material. A variety of different aerogel compositions are known and they may be inorganic, organic and inorganic/organic hybrid (see N. Hüsing and U Schubert, Angew. Chem. Int. Ed. 1998, 37, 22-45). Inorganic aerogels are generally based upon metal alkoxides and include materials such as silica, carbides, and alumina. Organic aerogels include, but are not limited to, urethane aerogels, resorcinol formaldehyde aerogels, and polyimide aerogels. Organic/inorganic hybrid aerogel were mainly organically modified silicate (organically modified silica or “ormosil”). The organic components are covalently bonded to the silica network. In other words, the organic and inorganic phase are chemically bonded to each other in the inorganic/organic hybrid aerogels.
Low-density aerogel materials (0.01-0.3 g/cc) are widely considered to be the best solid thermal insulators, better than the best rigid foams with thermal conductivities of 10 mW/m-K and below at 100° F. and atmospheric pressure. Aerogels function as thermal insulators primarily by minimizing conduction (low density, tortuous path for heat transfer through the solid nanostructure), convection (very small pore sizes minimize convection), and radiation (IR absorbing or scattering dopants are readily dispersed throughout the aerogel matrix). Depending on the formulation, they can function well at cryogenic temperatures to 550° C. and above. Aerogel materials also display many other interesting acoustic, optical, mechanical, and chemical properties that make them abundantly useful. The methods described in this invention represent advances in gel formations that will facilitate production and improved properties of these aerogel materials.
Low-density insulating materials have been developed to solve a number of thermal isolation problems in applications in which the core insulation experiences significant compressive forces. For instance, polymeric materials have been compounded with hollow glass microspheres to create syntactic foams, which are typically very stiff, compression resistant materials. Syntactic materials are well known as insulators for underwater oil and gas pipelines and support equipment. Syntactic materials are relatively inflexible and of high thermal conductivity relative to flexible aerogel composites (aerogel matrices reinforced by fiber). Aerogels can be formed from flexible gel precursors. Various flexible layers, including flexible fiber-reinforced aerogels, can be readily combined and shaped to give pre-forms that when mechanically compressed along one or more axes, give compressively strong bodies along any of those axes. Aerogel bodies that are compressed in this manner exhibit much better thermal insulation values than syntactic foams. Methods to improve performance of these materials such as density, thermal conductivity and dustiness will facilitate large-scale use of these materials in underwater oil and gas pipelines as external insulation.
Silica aerogel monolith will find use as insulating transparencies, such as double-glazing windows in buildings. Because these gel materials are normally stiff and inflexible when they are composed of a ceramic or cross-linked polymer matrix material with intercalated solvent (gel solvent) in the absence of fiber reinforcement, these materials need to be handled with great care.
Although the diffusion of polymer chains and subsequent solid network growth are significantly slowed within the viscous gel structure after the gelation point, the maintenance of the original gel liquid (mother liquor) for a period of time after gelation is essential to obtaining an aerogel that has the best thermal and mechanical properties. This period of time that the gel “ages” without disturbance is called “syneresis”. Syneresis conditions (time, temperature, pH, solid concentration) are important to the aerogel product quality.
Conventional methods for gel monolith and/or fiber-reinforced composite gel production formed via sol-gel chemistry described in the patent and scientific literature invariably involve batch casting. Batch casting is defined herein as catalyzing one entire volume of sol to induce gelation simultaneously throughout that volume. Gel-forming techniques are well-known to those trained in the art: examples include adjusting the pH and/or temperature of a dilute metal oxide sol to a point where gelation occurs (R. K. Iler, Colloid Chemistry of Silica and Silicates, 1954, chapter 6; R. K. Iler, The Chemistry of Silica, 1979, chapter 5, C. J. Brinker and G. W. Scherer, Sol-Gel Science, 1990, chapters 2 and 3). Suitable materials for forming inorganic aerogels are oxides of most of the metals that can form oxides, such as silicon, aluminum, titanium, zirconium, hafnium, yttrium, vanadium, and the like. Particularly preferred are gels formed primarily from alcohol solutions of hydrolyzed silicate esters due to their ready availability and low cost (alcogel).
It is also well known to those trained in the art that organic aerogels can be made from melamine formaldehydes, resorcinol formaldehydes, and the like (see for instance N. Hüsing and U Schubert, Angew. Chem. Int. Ed. 1998, 37, 22-45).
The availability of fiber reinforced aerogel composites opened up many application areas for aerogel materials. Since large pieces of aerogel composite materials have been successfully manufacture by this method, which can be widely used in all type of thermal and acoustic insulation applications. Yet it is inherently impossible to produce transparent aerogel composite, due to the presence of macro scale phase separation in these materials. A different reinforcement method is needed to produce stronger transparent aerogel monoliths, for the other insulation applications such as insulating transparencies in double glazing windows. In the past two decades, many investigators have attempted to improve the mechanical properties of silica in order to reduce its tendency to crack during the formation of its monoliths, by the incorporation of a secondly polymeric phase directly bonded to silica network. These led to the formations of numerous ormosil type of inorganic organic hybrid materials. Some of the most noticeable examples are as follows:
H. Schmidt, J. Non-Cryst. Solid, 73, 681, 1985, reported the incremental improvement of the mechanical properties of silica xerogel by the incorporation of PMMA or epoxy based polymer.
Mackenzie, et. al. J. Non-Crystalline solid 147&148 (1992), 271-279, J. Mater. Science, 27, (1992), 4415-4420, Mark, et al. Macromolecules, (1984), 11, 2613-2616, Macromolecules, 20, (1987), 1322-1330, O. Foussaier, M. Menetrier, J. Videau, E. Duguet, Mater. Lett. 42, 305, 2000, reported the improvement of the tensile properties of silica xerogel, by the incorporation of polydimethylsiloxane (PDMS) linear polymer.
H. Huang, G. L. Wilkes and J. G. Carlson, Polymer, 30, 1989, 2001-2012, reported the improvement on the tensile properties of silica xerogel by the incorporation of polyurethane linear polymer in the silioxane network.
It has been claimed that linear polymer such as PDMS appear to increase the flexible properties of the rigid silica aerogels. (S. J. Kramer, F. Rubio-Alonso and J. D. Mackenzie, MRS Proc. Vol 435, 295-300, 1996).
To distinguish between aerogels and xerogels, it is pointed out that aerogels are a unique class of materials characterized by their low densities, high pore volumes, and nanometer pore sizes. Because of the high pore volumes and nanometer pore sizes of aerogels, they typically have high surface areas and low thermal conductivities. The high porosity leads to a low solid thermal conductivity, and the nanometer pore sizes cause partial suppression of gaseous thermal conduction because the cells are smaller than the mean free path of gases. This structural morphology of an aerogel is a major advantage in thermal insulation applications. For instance, thermal conductivities have been measured to be less than 20 mW/m·K (J. Fricke and T. Tillotson, Thin Solid Films, 297 (1997) 212-223), and sometimes as low as 10-12 mW/m·K, at ambient conditions for silica aerogels. Thermal conductivities as low as 8-10 mW/m·K for organic aerogels (such as those composed of resorcinol-formaldehyde) have been measured. (R. W. Pekala and L. W. Hrubesh, U.S. Pat. No. 5,731,360). This is in sharp contrast to xerogels, which have higher densities than aerogels and are used as a coating such as a dielectric coating.
The sol-gel process has been used to synthesize a large variety of inorganic and hybrid inorganic-organic xerogels, aerogels and nanocomposite materials. Relevant precursor materials for silica based aerogel synthesis include, but are not limited to, sodium silicates, tetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS), monomeric alkylalkoxy silanes, bis trialkoxy alkyl or aryl silanes, polyhedral silsesquioxanes, and others. Various polymers have been incorporated into silica gels to improve mechanical properties of the resulting gels, xerogels (see J. D. Mackenzie, Y. J. Chung and Y. Hu, J. Non-Crystalline solid 147&148 (1992), 271-279; and Y. Hu and J. D. Mackenzie. J. Mater. Science, 27, (1992)), and aerogels (see S. J. Kramer, F. Rubio-Alonso and J. D. Mackenzie, MRS Proc. Vol 435, 295-300, 1996). Aerogels are obtained when the gels are dried in a manner that does not alter or causes minimal changes to the structure of the wet gel. This is typically accomplished by removing the solvent phase from the gel above the critical point of the solvent or mixture of solvents if a co-solvent is used to aid the drying process.
Wet gels frequently exhibit structures with mass fractal features consisting of co-continuous solid and pore liquid phases where the pore liquid phase can occupy as much as 98% of the sample volume. Aerogels have structures that are very similar to that of the original gel because they are dried by supercritical processes that minimize or eliminate capillary forces that cause the gel structure to collapse. The structure of xerogels, in contrast, is significantly modified during drying due to the capillary forces acting on the solid network during the evaporative drying process. The magnitude of the capillary pressure exerted on the solid network during evaporation is inversely proportional to pore dimensions (e.g. pore radius), and thus can be extremely large when pore features are in the nanometer (10−9 meters) range. These surface tension forces created during evaporative drying cause the gel network to fold or condense during xerogel manufacture as the coordination number of the particles increases.
Stated differently, a xerogel is formed upon conventional drying of wet gels, that is by increase in temperature or decrease in pressure with concomitant large shrinkage (and mostly destruction) of the initially uniform gel body. This large shrinkage of a gel body upon evaporation of the pore liquid is caused by capillary forces acting on the pore walls as the liquid retreats into the gel body. This results in the collapse of the filigrane, the highly porous inorganic network of the wet gels. Collapse of the structure stops when the gel network becomes sufficiently strong to resist the compressive forces caused by the surface tension.
The resulting xerogel typically has a close packing globular structure and no larger pores observable by TEM, which suggests that they are space filling. Thus the dried xerogel structure (which comprises both the skeletal and porous phases) is a contracted and distorted version of the original wet gel's structure. Because of the difference in drying procedures, xerogels and aerogels have very different structures and material properties. For instance, the number of reactive groups directly associated with a typical Si atom is significantly higher on average in an aerogel structure (dried supercritically) than in the corresponding xerogel structure made with the same starting formulation but dried evaporatively. Stated differently, the solutions or mixtures generally used to prepare a xerogel cannot be used to prepare an aerogel simply by altering the drying conditions because the resultant product will not automatically have a density of an aerogel. Thus there are fundamental compositional differences between xerogels and aerogels that greatly affects their surface area, reactivity, pore volume, thermal conductivity, compressibility, mechanical strength, modulus, and many other properties.
Thus compared to xerogels, aerogels are expanded structures that often more closely resemble to the structure of wet gel. TEM micrographs of aerogels often reveal a tenuous assemblage of clusters that bound large interstitial cavities. Porosity measurement by nitrogen sorption also reveals the structural difference in nanometer size level, compared to the corresponding xerogel, the aerogel often contains over twice the pore volume and average the pore size is considerably greater as is evident from the larger amount of adsorption that occurs at high relative pressures (>0.9). See C. J. Brinker and G. W. Scherer, Sol-Gel Science, 1990, Chapter 9. Due to the structural difference between aerogel and xerogels, there is significant difference in the physical properties of these two classes of materials, such as dielectric constant, thermal conductivities, etc. Therefore, and even if of identical elemental composition, an aerogel and its corresponding xerogel are completely different materials, somewhat analogous to sugar granules and cotton candy, both of which are composed of the same sugar molecules.
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